A lesion site generally has a different color from normal mucosal tissue. In recent years, improvements in the performance of color endoscope apparatuses have made it possible for an operator to identify and diagnose a lesion site whose color is slightly different from normal tissue. However, an operator needs extensive training under the guidance of an expert in order to be able to accurately distinguish a lesion site from normal tissue based on a slight color difference in an image captured by an electronic endoscope and then make a diagnosis. Also, even an experienced operator may not be able to easily identify and diagnose a lesion site based on a slight color difference, and this requires careful work.
In view of this, JP 2014-18332A (referred to hereinafter as “Patent Document 1”) is one example of a document that describes a device for scoring a lesion site that appears in a captured image in order to facilitate the diagnosis of a lesion site by an operator. Specifically, with the device described in Patent Document 1, the pixels that constitute an image captured by an electronic endoscope are subjected to tone enhancement processing for applying non-linear gain to the pixel values, the dynamic range is widened in the vicinity of the boundary of a region of pixel values that are to be subjected to lesion site determination, the tone-enhanced pixel data in an RGB space, which is defined by the three primary colors RGB, is converted to a predetermined color space such as the HIS color space or the HSV color space in order to acquire hue and saturation data, pixels are determined to be or not be lesion site pixels based on the acquired hue and saturation data, and then an evaluation value (lesion index) is calculated based on the number of pixels determined to be lesion site pixels.